Rh-Catalyzed Asymmetric Hydrogenation of β-Branched Enol Esters

Letter Cite This: Org. Lett. 2018, 20, 108−111

pubs.acs.org/OrgLett

Rh-Catalyzed Asymmetric Hydrogenation of β‑Branched Enol Esters for the Synthesis of β‑Chiral Primary Alcohols Chong Liu,† Jing Yuan,† Jian Zhang,‡ Zhihui Wang,‡ Zhenfeng Zhang,*,‡ and Wanbin Zhang*,†,‡ †

Shanghai Key Laboratory for Molecular Engineering of Chiral Drugs, School of Chemistry and Chemical Engineering, and ‡School of Pharmacy, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China S Supporting Information *

ABSTRACT: An asymmetric hydrogenation of β-branched enol esters has been developed for the first time, providing a new route for the synthesis of β-chiral primary alcohols. Using a (S)-SKP-Rh complex bearing a large bite angle and enol ester substrates possessing an O-fomyl directing group, the desired products were obtained in quantitative yields and with excellent enantioselectivities.

C

Scheme 1. Three Asymmetric Hydrogenations for the Synthesis of β-Chiral Primary Alcohols

hiral alcohols are versatile intermediates for the synthesis of various important pharmaceuticals and bioactive compounds, either directly or in the form of esters and ethers. Therefore, numerous methodologies have been developed for the asymmetric catalytic synthesis of chiral alcohols. However, most of these reports concern the construction of chiral secondary and tertiary alcohols, while the preparation of chiral primary alcohols has been less studied. Enzyme-catalyzed (dynamic) acylative kinetic resolution of racemic β-branched primary alcohols1 and dynamic reductive kinetic resolution of racemic α-branched aldehydes2 are the most attractive methodologies for the synthesis of such compounds with satisfactory enantioselectivities. Two indirect methods, asymmetric hydroboration/oxidation of 1,1-disubstituted alkenes3 and asymmetric carboalumination/or carboboration/oxidation of 1-substituted alkenes,4 have also been developed. However, several problems associated with these processes, such as low efficiency, high catalyst loading, complex reaction system, or large amounts of byproduct, render them impractical. Since its discovery, asymmetric hydrogenation (AH) has been considered to be a practical process because of its high efficiency, environmental friendliness, and low economic cost.5 According to retrosynthetic analyses, one can conclude that there are three routes of asymmetric hydrogenations for the synthesis of β-chiral primary alcohols (Scheme 1). One route involves the asymmetric hydrogenation of racemic α-branched aldehydes via dynamic kinetic resolution which has been developed independently by Zhou’s and List’s groups.6 Using a catalytic system of bisphosphine-Ru-diamine, high enantioselectivities were obtained for substrates bearing bulky α-alkyl substituents. However, high hydrogen pressure (20−50 atm) and a strong base (usually t-BuOK) are required to complete the reaction. A second route involves the asymmetric hydrogenation of β-branched allylic alcohols or esters which has been primarily studied by Andersson, Diéguez and co-workers using a PN−Ir catalytic system.7 However, only two types of substrates were used in order to obtain the β-chiral propanols and their acetate derivatives with 40−98% ee. Considering the limitations of the aforementioned routes, herein we propose a third route for the © 2017 American Chemical Society

synthesis of β-chiral primary alcohols via the asymmetric hydrogenation of β-branched enol esters. To the best of our knowledge, the asymmetric hydrogenation of enol esters bearing an α-branched substituent has been widely studied,8 but no report has been published concerning the asymmetric hydrogenation of enol esters bearing only a β-branched substituent (Figure 1), probably due to the difficulties in preparing pure substrates with only a Z- or E-configuration and developing active catalytic systems with good stereocontrol. The initial hydrogenation reactions were conducted on the model substrate (E)-2-phenylprop-1-en-1-yl acetate (1a-Me) which was synthesized by acetylation of 2-phenylpropanal. In order to utilize the directing effect of the acetyl group, rhodium complexes with the ability to chelate to enol esters were chosen. Several bisphosphine ligands bearing C2−4 backbones were Received: November 8, 2017 Published: December 8, 2017 108

DOI: 10.1021/acs.orglett.7b03469 Org. Lett. 2018, 20, 108−111

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Organic Letters

In order to investigate the effect of the acyl group on the reaction, we synthesized other substrates and subjected them to hydrogenation (Table 1). The enantioselectivities gradually Table 1. Effect of Acyl Groups

Figure 1. Enol esters for asymmetric hydrogenation.

evaluated. Irrespective of whether or not its chirality is central, axial, planar, or even P-stereogenic, almost all the commonly used chiral bisphosphine ligands gave the desired product with poor conversions (all 99 >99 >99 >99 99 >99 trace 73

93 90 85 76 nd 97 97 nd −92

a

Conditions: 1a-R (0.2 mmol), (S)-SKP (1.2 mol %), [Rh(nbd)2]BF4 (1.0 mol %), H2 (1 atm), DCM (2 mL), 25 °C, 5 h, unless otherwise noted. bThe conversions were calculated from 1H NMR spectra. cThe ee’s were determined by HPLC using chiral columns. d[Rh(cod)2]SbF6 was used instead of [Rh(nbd)2]BF4. e1 h. f(Z)-isomer was used instead of (E)-isomer. g5 atm.

Scheme 2. Screen of Ligands

decreased when the size of the acyl group increased (entries 1− 4). These results inspired us to explore a new directing group the O-formyl group. To avoid the difficult separation for the Zand E-isomers, a selective synthetic route for the preparation of the E-isomer has been developed.13 Methyl (E)-3-phenylbut-2enoates were selectively synthesized via Horner−Wadsworth− Emmons reaction using commercially available acetophenones and subsequently transformed to β,β′-disubstituted-(E)-vinyl formates via sequential DIBAL-H reduction, MnO2 oxidation, and Baeyer−Villiger oxidation (see Supporting Information for details). However, the hydrogenation using the same rhodium salt with a BF4 anion showed poor activity for this substrate (entry 5). Therefore, a more active rhodium salt bearing a SbF6 counterion was utilized and gave the desired product in quantitative conversion and 97% ee under the same reaction conditions or even in a short reaction time of 1 h (entries 6 and 7). The hydrogenation of the (Z)-isomer (R = H) was also conducted under 1 atm of hydrogen pressure, but almost no product was detected (entry 8). When increasing the hydrogen pressure to 5 atm, the hydrogenated product was obtained with 73% conversion and with an opposite enantioselectivity of 92% ee (entry 9). With the optimized reaction conditions in hand, we investigated the substrate scope of the hydrogenation reaction catalyzed by the isolated rhodium complex [Rh((S)-SKP)(cod)]SbF6 (Scheme 3). All substrates bearing different phenyl moieties gave their corresponding products 2a−l in satisfactory yields and excellent enantioselectivities (91−99%), regardless of the presence of electron-withdrawing or -donating substituents. The 1- and 2-naphthyl group-substituted substrates showed the best results giving their corresponding products 2m and 2n with 99% and 97% ee’s, respectively. Furthermore, substrates bearing an alkyl unit at the β-position furnished their related products 2o−r with excellent results, irrespective of the steric size of the substituted group. To gain insight into the reaction mechanism, a deuteriumlabeling experiment was conducted. The use of D2 instead of H2

DuPhos analogue bearing a ferrocene backbone ((R,R)-MeFcPhos) showed a dramatically improved activity. However, the enantioselectivity was still not acceptable (Scheme 2). This result suggested that ligands bearing a large bite angle appear to be more active in this reaction. A similar trend was also observed in our previous work concerning the asymmetric hydrogenation of β-branched allylic alcohols.9 Thus, three chiral bisphosphine ligands, (R)-PhanePhos,10 (S)-CH2OH-DPPF,11 and (S)-SKP12 possessing a large bite angle, were tested. To our delight, all of these ligands gave the desired product with complete conversion and improved enantioselectivities. In particular, (S)-SKP, which was developed by Ding et al.,12 was found to be the most promising (Scheme 2). Attempts at hydrogenation of an (E/Z)mixture (45/55) under 30 atm of hydrogen pressure only gave a racemic product (−6% ee), indicating that the (E)- and (Z)isomers generate the desired product with opposite enantioselectivity. Further optimization of the reaction conditions, such as solvent, temperature, and hydrogen pressure, did not provide any obvious improvement for this reaction (see Table S1 in Supporting Information). It is noteworthy that this reaction can go to completion under 1 atm of hydrogen pressure within a few hours. 109

DOI: 10.1021/acs.orglett.7b03469 Org. Lett. 2018, 20, 108−111

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Organic Letters Scheme 3. Substrate Scopea

enantioselectivity (Scheme 4b). Additionally, we examined the hydrogenation of substrate 1t bearing a heteroaromatic 2-thienyl group (regarded as a coordinating substituent) and obtained the hydrogenated product 2t with reduced activity and enantioselectivity (Scheme 4c). To demonstrate the practicality of this methodology, the hydrogenated products were transformed to useful bioactive molecules (Scheme 5). For instance, compound 2h was Scheme 5. Applications of Hydrogenated Products

hydrolyzed to the corresponding alcohol 3h and then reacted with mesyl chloride to give a glutamate receptor potentiator 4h.15 The product 2o underwent hydrolysis in aqueous NaOH to give (S)-2-phenyl-1-butanol (3o), which can be transformed to an enantiomer of an orally active TAAR1 agonist 4o.16 In conclusion, based on a new strategy that allows access to substrates as a single E-isomer, and the development of an efficient system to control the enantioselectivity by using the bisphosphine ligand bearing a large bite angle and substrates possessing an O-fomyl directing group, β-branched enol esters have been hydrogenated for the first time to give β-chiral primary alcohols quantitatively and with excellent enantioselectivities.

a

Conditions: 1 (0.2 mmol), [Rh((S)-SKP)(cod)]SbF6 (1 mol %)], DCM (2 mL), H2 (1 atm), 25 °C, 1 h, unless otherwise noted. Isolated yields. The ee’s were determined by HPLC using chiral columns. b6 h. c 30 atm, 24 h. dDetermined after hydrolysis. e40 atm, 24 h.

resulted in deuterium addition to the alkene with no doublebond-shift occurring (Scheme 4a). This result, combined with

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ASSOCIATED CONTENT

S Supporting Information *

Scheme 4. Mechanism Study

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b03469. Synthetic details for substrates, procedures for hydrogenation reactions, spectra of NMR and HPLC data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhenfeng Zhang: 0000-0003-3295-6966 Wanbin Zhang: 0000-0002-4788-4195

the above-mentioned hydrogenation performance of 2a and 2o− r that no decrease in ee value was observed following an increase in the size of the alkyl chain, and that the (E)- and (Z)-isomers generate products with the opposite enantioselectivity, reveals that the stereoselectivity is predominantly controlled by the chelating direction of the enol ester to the bisphosphine− rhodium−dihydride complex, and not because of the diffenence between the two β-substituents. These results are consistent with the reported mechanism concerning the asymmetric hydrogenation of enamides.14 Additional experiments that were also conducted may support this evidence to some extent. A substrate with two similar β-aromatic groups (1s) was hydrogenated to give the desired product 2s with complete conversion and good

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by National Natural Science Foundation of China (Nos. 21232004, 21572131, and 21620102003), Shanghai Municipal Education Commission (No. 201701070002E00030), and Science and Technology Commission of Shanghai Municipality (No. 15JC1402200). We thank the Instrumental Analysis Center of Shanghai Jiao Tong University. 110

DOI: 10.1021/acs.orglett.7b03469 Org. Lett. 2018, 20, 108−111

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DOI: 10.1021/acs.orglett.7b03469 Org. Lett. 2018, 20, 108−111